Before FDR blurted out in the Casablanca press conference that nothing but “Unconditional Surrender” would be accepted as an end to WW2 nobody in the entourage including Churchill had acquiesced to that idea. Indeed, it had not even been discussed.
The Military men knew that such a proposal meant that the War would now be extended as the Germans were now prompted to fight with the despairing ferocity like cornered rats.
One British Government advisor (Lord Maurice Hankey) took the time to look at the history back to 1600 (covering 15 British wars) of Unconditional Surrender as a condition for a war’s end. He found only one example and that was the Boer War and it didn’t work then and was given up when the Brits learned from the Boers that they would fight till Kingdom Come as opposed to Unconditionally surrender.
The only noteworthy example that Hankey found of Unconditional Surrender being invoked and accomplished was the Third Punic War. The Romans demanded the Carthaginians Unconditional Surrender and the Carthaginians told them to pound sand resulting in Carthage being razed to the ground once Rome finally conquered.
But Rome had planned to raze Carthage Unconditional Surrender or not.
Where did FDR come up with this idea and why blurt it out for the first time in a press conference at the end of a meeting with Churchill on the tactical and strategic priorities of WW II? Was FDR speaking for Stalin here? FDR was too much of a politician for anyone to believe he just unconsciously just blurted it out.
However, I have no problem believing that Trump is clueless about what he has put into motion by demanding Unconditional Surrender.